1. Field of the Invention
The present invention relates to communication systems, and more particularly, to a passive echo cancellation device and method.
2. Description of the Prior Art
With the progress of technologies, more and more applications of the Internet have been developed. In view of the growing needs for the bandwidth of networking, the packet transmission speed of the extensively used Ethernet has be enhanced from the previous speed of 10/100 Mbps to the current speed of more than 1 Gbps.
For example, in a commonly seen Gigabit Ethernet device having a transmission speed of 1 Gbps, each port thereof comprises four channels. Wherein, each channel has a transceiver for communicating with other network devices through a transmission medium such as a twisted pair cable. Further, each transceiver typically includes a transmitting end and a receiving end. The transmitting end is provided for processing data and then transmitting the processed data to a remote network device through the transmission medium, while the receiving end is provided for receiving and processing the data transmitted through and from the transmission medium. According to Gigabit Ethernet standard, each device communicates with other network devices by using the four channels simultaneously wherein each of the channels simultaneously performs data transmitting and receiving. In other words, the Gigabit Ethernet is a full duplex communication system.
The characteristic of a full duplex communication system such as the Gigabit Ethernet lies in that each of the channels simultaneously performs data transmitting and receiving. However, in such a system, when the network device is acquiring a signal Rx from one of the channels, a signal Tx simultaneously transmitted through the same channel may incur interference upon the received signal Rx. As a result, the integrity of the received signal is unrecognizable and this is generally referred to as echo impairment. In the attempt to minimize the effect of echo impairment, an echo cancellation circuit is commonly used in a network device so as to obviate the components of the transmitted signal from the signal the device receives.
Known echo cancellation circuits are typically equipped with active elements such as operational amplifiers and transistors. By using such active elements, an echo cancellation circuits can not only obviate the echo effects but also actively provide gain to the signal it receives. Nevertheless, the implement of these active elements brings the disadvantages of relatively complex construction, higher manufacturing costs and greater power consumption to the circuits.